Problem: Solve for $a$ : $2 = 4 + a$
Explanation: Subtract $4$ from both sides: $ \begin{eqnarray} \\ \hphantom{2 {- 4}} &\hphantom{=}& \hphantom{a} \\ 2 &=& 4 + a \\ \\ {-4} && {-4} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 2 {- 4} &=& a \\ \end{eqnarray} $ Simplify. $ a = -2$